Griefer
A person who purposely tries to annoy or anger other players.
Grinding
Repeating a task over and over to achieve a goal. Generally refers to something that takes a long time, such as gaining experience, faction reputation, skill points, or honor.
Gruul
The second and final boss encounter of Gruul's Lair, the first 25-man raid instance in the Burning Crusade
HK
Honorable Kill, when a player kills another player who yields honor.
HP
Health, Health Points, Hit Points. When these are reduced to 0, the character or NPC dies.
HS
Healthstone, a warlock conjured item that restores health to the user
IDK
I Don't Know
IF
Ironforge, home of the Dwarves
IMHO
In My Humble Opinion
IMO
In My Opinion
Inc
Incoming. This means enemies are coming. Used when pulling or to warn others of adds or patrols.
Instance
A dungeon where you will load into your own copy of the dungeon with your group. Only you and your group will be in your copy of the dungeon. Another group that enters the same area will enter their own copy of the dungeon.
INT
Intelligence, a base statistic
IRL
In Real Life
Kara
Karazhan, a 10-player raid and the first level 70 raid instance. Located in Deadwind Pass.
Kite
A technique for fighting which involves staying out of range of an enemy while simultaneously doing damage to it
LFG
Looking for group
LFM
Looking for more
LOL
Laugh out loud
LOM
Low on Mana